Over-limit electrical condition protection circuits and methods

ABSTRACT

Apparatuses and methods for protecting a circuit from an over-limit electrical condition are disclosed. One example apparatus includes a protection circuit coupled to a circuit to be protected. The circuit to be protected is coupled to a pad node. The protection circuit is configured to conduct current from the pad node to a reference voltage node to protect the circuit from an over-limit electrical condition. The protection circuit has a trigger circuit coupled to the pad node and configured to trigger a shunt circuit to conduct current from the pad node to the reference voltage node responsive to a voltage provided to the pad node having a voltage exceeding a trigger voltage. In some embodiments, the trigger circuit is matched to the circuit being protected.

TECHNICAL FIELD

Embodiments of the invention relate generally to integrated circuits,and more particularly, in one or more of the illustrated embodiments, toprotection circuitry for over-limit electrical conditions that maydamage the integrated circuits.

BACKGROUND OF THE INVENTION

Integrated circuits are connectable to “the outside world” through inputnodes, output nodes, or input/output nodes such as bond pads, inputpads, input/output pins, die terminals, die pads, or contact pads.Circuitry is often interposed between such nodes and active circuitry ofthe integrated circuit. The circuitry typically includes transistorswhich should be protected from over-limit electrical conditions. Thismay be especially true for circuits that include field-effecttransistors (FETs), which are formed having a gate insulator. Anuncontrolled over-limit electrical event may subject the gate insulatorto a relatively high voltage that exceeds a breakdown voltage thatcauses permanent damage to the transistor.

An electrostatic discharge event during which the circuitry is subjectedto an electrostatic discharge (ESD) is an example of an over-limitelectrical condition that may cause damage to the circuitry of theintegrated circuit unless adequately protected. Another example of anover-limit electrical condition for example, latch-up, may result froman “overdrive condition,” An overdrive condition exists when voltages orcurrents at an electrical node exceed specified levels, such as amanufacturer's specification of the “normal” operating parameters forthe device. Overdrive conditions can be contrasted with what istypically referred to as a normal operating conditions, that is,conditions specified by a semiconductor device manufacturer to be withinspecified limits. Circuitry subjected to an overdrive condition mayconduct current inadvertently and without control over the current pathor the magnitude of current conducted. It is desirable, however, thatcircuitry is designed to withstand an occasional or even sustainedoverdrive condition without adverse consequences. Uncontrolled overdriveconditions, in contrast, may cause over-limit electrical conditions thatmay damage circuitry, and consequently, should be avoided.

Typically, an over-limit protection circuit is connected to a node, suchas a bond pad, that may be subjected to an over-limit electricalcondition in order to protect circuitry also coupled to the node.Typical over-limit electrical condition protection circuits includecircuitry that provide a low-impedance conductive path from the node toa reference voltage, such as ground, to dissipate the over-limitelectrical condition before operational circuitry also coupled to thenode are damaged. For example, the over-limit protection circuit keepsthe potential of the bond pad from exceeding a maximum value.

Many of the protection circuits include circuits that exhibit a“snap-back” characteristic. Generally, a snap-back characteristicprovides a trigger condition which when exceeded, causes the circuit toenter a low-impedance state. The low-impedance state is maintained whilethe electrical condition on a node exceeds a hold condition. Indesigning an adequate protection circuit using a snapback circuit, thetrigger condition for the snapback circuit must be appropriate for theelectrical conditions the node will experience under normal operatingconditions. For example, the trigger conditions should be sufficientlyhigh to prevent the protection circuit from inadvertently triggering butlow enough to trigger before operational circuitry coupled to the nodeare subjected to damaging over-limit electrical conditions. An exampleof a node that will be subjected to relatively high voltages duringnormal operation are high-voltage (HV) pads which are used to providecircuitry relatively high-voltages during normal operation. Over-limitprotection circuitry for such pads should be designed to avoidtriggering when the expected operating voltage is provided to the padbut nonetheless trigger at an over-limit electrical condition below thatwhich will damage circuitry coupled to the pad.

Examples of conventional circuits having snapback characteristicsinclude thyristors, such as silicon controlled rectifiers (SCRs), andoverdriven metal-oxide-semiconductor (MOS) transistors, and diodes.Examples of conventional circuits having a set trigger condition, andtypically a set hold condition as well, include diode-triggered SCRs(DTSCRs). Once set, however, adjusting (e.g. changing, altering, etc.)the trigger condition often requires redesign of the protection circuit.That is, the protection circuits are typically “hard-wired” and are notmodified after the integrated circuit is fabricated. Moreover, triggerconditions for ESD protection and protection against latch-up conditionsare often different, thus, having a protection circuit having a triggercondition set to protect against one condition may be a compromise forprotecting against the other over-limit electrical conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a protection circuit according to anembodiment of the invention.

FIGS. 2A-2D are schematic diagrams of protected circuits.

FIG. 3 is a current-voltage diagram of I-V curves exhibiting snapbackcharacteristics according to an embodiment of the invention.

FIG. 4 is a schematic diagram of a shunt circuit according to anembodiment of the invention.

FIG. 5 is a schematic diagram of a protection circuit according to anembodiment of the invention.

FIG. 6 is a cross-sectional diagram of the protection circuit of FIG. 5according to an embodiment of the invention.

FIG. 7 is a schematic diagram of a control circuit for a protectioncircuit according to an embodiment of the invention.

FIG. 8 is a schematic diagram of a protection circuit according to anembodiment of the invention.

FIG. 9 is a block diagram of a protection circuit according to anembodiment of the invention.

FIG. 10 is a block diagram of a memory including a protection circuitaccording to an embodiment of the invention.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without these particular details. Moreover, the particularembodiments of apparatuses and methods described herein are provided byway of example and should not be used to limit the scope of theinvention to these particular embodiments. In other instances,well-known circuits, control signals, timing protocols, and softwareoperations have not been shown in detail in order to avoid unnecessarilyobscuring the invention.

FIG. 1 illustrates a over-voltage protection circuit 100 according to anembodiment of the invention. The protection circuit 100 is coupled to aprotected circuit, for example, protected circuit 10, and protect theprotected circuit 10 from being subjected to an over-voltage condition,which as previously discussed, may otherwise damage the circuitry of theprotected circuit 10. As illustrated in FIG. 1, the protected circuit 10is coupled to pad 20 and a cathode node 30. The cathode node 30 may becoupled to a reference voltage, for example, ground. An over-voltagesignal may be inadvertently applied to the pad 20, in response to whichthe protection circuit 100 is activated and provides a current path tothe reference voltage. In the embodiment of FIG. 1, the protectioncircuit 100 is coupled to the pad 20 and the cathode node 30.

The protection circuit 100 includes a shunt circuit 110 and a triggercircuit 140. The trigger circuit 140 is configured to trigger the shuntcircuit 110 to provide a current path responsive to an over-voltagebeing applied to the pad 20 through which current resulting from theover-voltage may be shunted to protect the protected circuit 10 frombeing damaged. Additionally, the shunt circuit 110 is configured to holda voltage across the protected circuit 10 to below a voltage theprotected circuit 10 may be subjected before being damaged.

Protected circuits 10 may include one or more transistors. Examples ofvarious protected circuits 10 are illustrated in FIGS. 2A-2D. FIG. 2Aillustrates a driver 210 having one or more transistors 220(1)-220(n)that are used to provide an output signal OUT responsive to respectiveinput signals Ng_1-Ng_n. The transistors 220(1)-220(n) are illustratedin the embodiment of FIG. 2A as n-channel field effect transistors(nFETs). FIG. 2B illustrates a driver 230 having a one or moretransistors 240(1)-240(n) that are used to provide an output signal OUTresponsive to respective input signals Pg_1-Pg_n. The transistors240(1)-240(n) are illustrated in the embodiment of FIG. 2B as p-channelfield effect transistors (pFETs). The OUT signal may have a relativelyhigh-voltage provided to the pad 20.

FIG. 2C illustrates a logic circuit 250 having a pull-up transistor 254and a pull-down transistor 258. An output signal OUT is provided by thelogic circuit 250 responsive to input signal IN. In the embodiment ofFIG. 2C, the pull-up transistor 254 is illustrated as a pFET and thepull-down transistor 258 is illustrated as an nFET. FIG. 2D illustratesa voltage detect circuit 260 having diode-coupled transistors270(1)-270(n) series coupled between the pad 20 and a gate of a detecttransistor 280. A detection output signal DETOUT based on a detectioninput signal DETIN may be provided by the detect circuit 260 responsiveto a voltage applied to the pad 20 exceeding a voltage. Thediode-coupled transistors 270(1)-270(n) are illustrated in theembodiment of FIG. 2D as diode-coupled nFETs, and the detect transistor280 is illustrated as a nFET. Transistors of different types than thoseshown in the embodiments of FIGS. 2A-2D may be used as well. Thecircuits of FIGS. 2A-2D have been provided by way of example and theinvention is not limited to the specific examples of protected circuits10 described.

Returning to FIG. 1, the shunt circuit 110 includes at least a portionthat is formed in a p-well in which at least a portion of the triggercircuit 140 is formed, which will be described in more detail below.Leakage currents from the trigger circuit 140 resulting from anover-voltage condition are used to enhance forward biasing of a diodejunction of the shunt circuit 110, and thus, trigger the shunt circuit110. The p-well is formed in a semiconductive material, such as asemiconductor substrate. As used herein, the term semiconductivematerial, includes a bulk semiconductive region, an epitaxial layer, adoped well region, and the like.

In some embodiments, the trigger circuit 140 may trigger the shuntcircuit 110 at a plurality of different trigger conditions. A controlcircuit 160 may be coupled to the shunt circuit 110 and trigger circuit140 and a reference voltage node (e.g., ground) to adjust the triggerconditions for the protection circuit 100. The trigger conditions may beset responsive to the control signal CNTRL.

A shunt circuit 110 exhibiting a “snapback” current-voltage (I-V)characteristic may be used to provide the protection circuit 100 withthe same characteristic. A protection circuit 100 having the snapbackcharacteristic is triggered at a trigger condition to provide a currentpath to shunt over-voltage current. Once triggered, a voltage across theprotection circuit 100 decreases to a hold condition having a voltagethat is less than a voltage for the trigger condition. The lower voltageof the hold condition protects the protected circuit 10 from beingdamaged by an over-voltage that exceeds the maximum voltage capabilityof the protected circuit 10.

FIG. 3 illustrates snapback characteristics for two conditions of aCNTRL signal applied to the trigger circuit 140 of FIG. 1. Inparticular, a first I-V curve 310 represents the response of theprotection circuit 100 for a CNTRL signal having a first voltage and asecond I-V curve 320 represents the response of the protection circuit100 for a CNTRL signal having a second voltage. Each of the I-V curvesexhibits a respective trigger condition, trig1 and trig2, and a holdcondition hold1 and hold2. The trigger and hold conditions representcurrent-voltage conditions to trigger the snapback response of theprotection circuit 100 and to maintain the snapback condition. Circuitshaving the general snapback response as illustrated in FIG. 3 that maybe used for the shunt circuit 110 of the protection circuit 100 areknown by those ordinarily skilled in the art. Such shunt circuits 110may be implemented using conventional snapback circuits or snapbackcircuits later developed.

The I-V curves of FIG. 3 may be used to illustrate an examplerelationship of the protection circuit 100 where the CNTRL signal isused to control the control circuit 160 to adjust added resistancethrough the control circuit 160. The I-V curve associated with aCNTRL=V2 exhibits a greater trigger condition (trig2) as well as agreater hold condition (hold2) relative to the I-V curve associated witha CNTRL=V1 having trigger condition trig1 and hold condition hold1. Theincrease in the trigger and hold conditions from trig1/hold1 totrig2/hold2 result from decreasing the added resistance through thecontrol circuit 160.

In some embodiments of the invention, the control circuit 160 is usedduring power-up of an integrated circuit in which the protection circuitis included. For example, when the integrated circuit having anembodiment of the invention is unpowered, the CNTRL signal controls thecontrol circuit 160 to add a relative large resistance between thetrigger circuit 140 and the reference voltage. As previously discussed,under this condition, the trigger voltage may be lowered for theprotection circuit 100. An advantage to a lowered trigger voltage isthat it will provide greater over-voltage/over-current protection to theprotected circuit 10 in the event a relatively high-voltage and/orcurrent is applied to the pad 20. That is, less voltage and/or currentis necessary to trigger the protection circuit 100 to discharge theover-voltage/over-current. An example of an event that presentsrelatively high-voltage and/or current to a node is an ESD impulse.

Following power-up of the integrated circuit, the CNTRL signal isadjusted to control the control circuit 160 to reduce the additionalresistance provided by the control circuit 160. As previously described,the decrease in resistance between the trigger circuit 140 and thereference voltage results in an increase to the trigger condition andthe hold condition of the protection circuit 100. The increased holdcondition increases latch-up immunity of the protection circuit 100. Insome embodiments, the CNTRL signal is adjusted to increase the holdcondition to approximately two-three times the operating voltage of theintegrated circuit. For example, where the operating voltage for anintegrated circuit is 1.0 V, the control circuit 160 is adjusted toprovide a hold condition approximately 2.0-3.0 V. As previouslydescribed, the CNTRL signal can be adjusted to modulate the performancecharacteristics of the protection circuit 100 to provide the desiredhold condition. The increased hold condition may prevent the ESD devicesfrom triggering based on acceptably normal power spikes that may occurduring certain operation cycles. If the power spike has relatively highvoltage and/or current levels that can induce damages to the integratedcircuits, or if there is an ESD event, the control circuits may capturethese changes, and switch mode to low trigger/hold voltage levels.

As described by the previous example, operating the protection circuit100 through the use of the control circuit 160 in such a manner canprovide both the relatively high voltage requirements to preventlatch-up and the relatively low trigger-current needed for ESDprotection. In other embodiments, the control circuit 160 is notoperated in a binary-type manner of providing either maximum addedresistance or minimum resistance. The control circuit 160 may beadditionally or alternatively adjusted continuously over the range ofthe available impedance using the CNTRL signal. In this manner, theadded resistance, and consequently, the trigger condition for theprotection circuit 100, can be adjusted to a desired level within theavailable range of modulation provided by the control circuit 160.

A shunt circuit 400 according to an embodiment of the invention isillustrated in FIG. 4. The shunt circuit 400 may be used as the shuntcircuit 110 of the protection circuit 100 of FIG. 1. The shunt circuit400 is a thyristor, such as a silicon controlled rectifier (SCR). Asknown and as illustrated in FIG. 4, an SCR is formed by a combination ofPNP-NPN bipolar junction transistors (BJTs) 410, 420. The resultingcircuit may be represented as three diodes 440, 450, and 460 coupledbetween the pad 20 and cathode node 30, as illustrated in FIG. 4. Thediode 460 may be integrated with trigger circuit by being formed in acommon well, as will be described in more detail below. An exampleconventional design includes formation of the PNP-BJTs and NPN-BJTs 410,420 in a p+-region formed in an n-well (i.e., a well region of n-typedoping) and a n+-region in p-well (i.e., a well region of p-typedoping). In embodiments of the invention utilizing the shunt circuit 400as the shunt circuit 110, a lateral NPN-BJT 420 may be formed in anp-well in which a portion of the trigger circuit is formed as well.

In operation, the shunt circuit 400 is triggered as the base-to-emitterdiode of the lateral NPN-BJT 420 is forward biased. Using conventionaldesigns, the forward bias for the base-to-emitter diode may beapproximately 0.6 V at room temperature. The base-to-emitter diode maybe forward biased as the voltage of the p-well in which the NPN-BJT 420is formed increases as a result of current discharging through theinherent resistance of the p-well. Current may be provided when thevoltage across the shunt circuit 400 causes a reverse-bias breakdown ofthe junction between the n-well in which the PNP-BJT 410 is formed andthe p-well in which the NPN-BJT 420 is formed. The typical breakdownvoltage for the nwell-pwell junction can be approximately 20 V. Currentthrough the p-well may also be provided from circuits of the triggercircuit formed in the same p-well as the NPN-BJT 420, as will bedescribed in more detail below. That is, leakage currents from thetrigger circuit resulting from an over-voltage condition may be used toforward bias the base-to-emitter diode of the NPN-BJT 420 of the shuntcircuit 400.

FIG. 5 illustrates a protection circuit 500 according to an embodimentof the invention. The protection circuit 500 is coupled to a protectedcircuit 50, which is shown in the embodiment of FIG. 5 as a high-voltagedriver circuit that includes one or more transistors 60(1)-60(n). Theprotected circuit 50 and the protection circuit 500 are coupled to a pad20 and a cathode node 30. The protection circuit 500 includes a shuntcircuit 510 and a trigger circuit 540.

In the embodiment of FIG. 5, the trigger circuit 540 is provided by adriver circuit that matches the driver circuit of the protected circuit50. The trigger circuit 540 includes one or more transistors550(1)-550(n) coupled in series between the pad 20 and the cathode node30. Each of the transistors 550(1)-550(n) correspond to a respective oneof the one or more transistors 60(1)-60(n) of the protected circuit 50.Transistors 550 have gates coupled to a respective one of transistors60, except for the last transistor 550(n). Transistors 550 and 60 (otherthan the last transistor 550(n)) have gates coupled together and biasedto minimize channel and gate-induced leakage currents. The gate oftransistor 550(n) and body regions of the transistors 550(1)-550(n) arecoupled to a control circuit 560. The control circuit 560 is coupled toa reference voltage, for example, ground. The transistor 550(n) remainsin an “off” state due to the coupling of its gate to ground through thecontrol circuit 560. The shunt circuit 510 includes a SCR having ananode node 512 and first base node 514 coupled together, and a secondbase node 516 coupled to the body regions of the transistors550(1)-550(n).

The control circuit 560 is represented by an adjustable resistance inthe embodiment of FIG. 5. The control circuit 560 may be used to setdifferent voltages at which the trigger circuit 540 triggers the shuntcircuit 510. The adjustable resistance of the control circuit 560 may beused to adjust resistance between the reference voltage and the bodyregions of the transistors 550(1)-550(n) and base node 516. Generally, alower resistance added by the control circuit 560 results in a highertrigger voltage than for a higher control circuit resistance. Providingthe control circuit 560 allows for base modulation of the SCR of thetrigger circuit 510, which can be used to adjust the trigger conditionsof the protection circuit 500.

Using a trigger circuit 540 that matches the circuitry of the protectedcircuit 50 provides a benefit that trigger circuit 540 will typicallytrigger the shunt circuit 510 earlier than when the protected circuit 50begins to react to an over-voltage applied to the pad 20, therebypreventing the protected circuit 50 from being damaged by theover-voltage. Trigger circuits matching the various protected circuitsillustrated by FIGS. 2A-2D may be used in protection circuits coupled toa corresponding protected circuit. In other embodiments of theinvention, however, the trigger circuit may not match the protectedcircuit.

In other embodiments of the ESD protection, trigger circuit 540 andprotected circuit 50 may be fully or partially merged. A portion of allof the transistors 60(1)-60(n) of the protected circuit 50 may replacethe transistors 550(1)-550(n) to trigger the shunt circuit 510. Whentransistors 60(1)-60(n) replace the original trigger transistors550(1)-550(n), the control circuit may still be used to control thep-well 540.

Portions of the shunt circuit 510 and the trigger circuit 540 may beformed in the same p-well (e.g., an isolated p-well). For example, thediode 460 (not shown in FIG. 5) may be formed in the same p-well as thetransistors of the trigger circuit 540. The trigger circuit 540 mayprovide leakage currents under an over-voltage condition that are usedto forward bias the diode 460 of the shunt circuit 510. FIG. 6illustrates a cross-sectional drawing of the shunt circuit 510 andtrigger circuit 540 according to an embodiment of the invention. Theprotected circuit 50 is also illustrated in FIG. 6.

A deep n-well 610 is formed in which p-well 630 and n-well 608 areformed. Portions of the shunt circuit 510 and the trigger circuit 540are formed in the p-well 630. The protected circuit 50 is formed inp-well 640 by n-regions 52, 54, and 56, as well as, gates 58 and 59formed over the p-well 640. The n-region 52 is coupled to cathode node30 and the n-region 56 is coupled to pad 20.

The SCR of the shunt circuit 510, which as previously discussed,includes back-to-back PNP-BJT and NPN-BJT (e.g., PNP-BJT 410 and NPN-BJT420 of FIG. 4). A PNP-BJT 410 is formed by p-region 612, the n-well 608,and p-well 630. An NPN-BJT 420 is formed by n-well 608, p-well 630, andn-region 632. An n-region 614 is used to couple the p-region 612 (i.e.,anode of PNP-BJT 650) to the n-base/n-collector of n-well 608. Anexternally accessible pad, for example, pad 20, is coupled to thep-region 612. The n-region 632 may be used to couple with the cathode ofthe SCR of the shunt circuit 510.

The trigger circuit 540 is formed by n-regions 632, 634 636, formed inthe p-well 630, and gates 650 and 652 formed over the p-well. Inparticular, with reference to FIG. 5, the transistor 550(1) is formed byn-regions 634, 636, and gate 652, and the transistor 550(n) is formed byn-regions 632, 634, and gate 650. The n-region 636 also provides forcoupling the trigger circuit 540 to the pad 20. Body regions of thetransistors 550(1)-550(n) and the p-base of the NPN-BJT 660 arerepresented by p-well 630. A p-region 638 provides an electricalconnection to the p-base of the NPN-BJT 660 and the body of transistors550(1)-550(n) of p-well 630. As known, a parasitic p-well resistance,represented in FIG. 6 as Rpwell 639 is present between the p-base of theNPN-BJT 420 and the p-region 638. The p-region 638 is coupled to acontrol circuit 670 (e.g., control circuit 560) and to gate 650 of thetransistor 550(n), as shown in FIG. 5. The control circuit 670 is shownschematically in FIG. 6. Those ordinarily skilled in the art, however,have sufficient knowledge to form the control circuit 670 and to couplethe control circuit 670 to p-well 630 and to gate 650.

In operation, under normal operating conditions the trigger circuit 540remains biased in an inactive state by the coupling the gate oftransistor 550(1) to the gate of transistor 60(1), as well as couplingthe gate of transistor 550(n) to a reference voltage (e.g., ground).When an over-voltage condition occurs, for example, an ESD event whichapplies relatively high voltage to the pad 20, a reverse-bias leakageconducts current from the n-region 636 (i.e., a drain of the triggercircuit 540) to the p-well 630, and from the p-well through the controlcircuit 670 to the reference voltage. Due to the Rpwell 639 and theresistance of the control circuit 670, the voltage of the p-well 630increases. The increase in voltage of the p-well in effect forwardbiases the emitter-base of the NPN-BJT 420, which causes the SCR of theshunt circuit 510 to conduct from the pad 20 to the reference voltage.The additional current provides positive feedback to further forwardbias the emitter-base. The voltage at the pad 20 is also controlled to amagnitude that is generally below the voltage at which the triggercircuit 540 triggered the SCR of the shunt circuit 510. Thus, theprotected circuit 50 is protected by maintaining the voltage below thetrigger voltage and shunting resulting from an ESD event through the SCRof the shunt circuit 510 rather than through the protected circuit 50.

The adjustable resistance of the control circuit 670 may be used to setdifferent voltages at which the trigger circuit 540 triggers the shuntcircuit 510. A lower resistance provided by the control circuit 670effectively results in a higher trigger voltage than for a highercontrol circuit resistance, or conversely, a higher resistance providedby the control circuit 670 effectively results in a lower triggervoltage for a lower control circuit resistance. That is, the p-wellcurrent required to forward bias the base-to-emitter of the NPN-BJT 420is decreased with a higher resistance, thereby effectively decreasingthe trigger voltage of the protection circuit 500.

FIG. 7 illustrates a control circuit 700 for a protection circuitaccording to some embodiments of the invention. As previously described,control circuits 560 and 670 may be used to set different voltages atwhich a trigger circuit triggers a shunt circuit. The control circuit700 may be used for the control circuits 560 and 670. The controlcircuit 700 includes a resistance 710 and a transistor 720 coupled inparallel to the resistance 710. The transistor 720 is controlled by acontrol signal CNTRL. The resistance 710 is illustrated in FIG. 7 as aresistor Rcontrol and the transistor 720 is illustrated as a nFET. Insome embodiments of the invention, the resistance 710 and the transistor720 may be implemented using other circuits.

In operation, the transistor 720 is used to adjust the overallresistance between a reference voltage to which a trigger circuit iscoupled and a p-well region in which at least a portion of a shuntcircuit and at least a portion of a trigger circuit are formed. Theresistance between the p-well and the reference voltage may be adjustedbased at least in part on the CNTRL signal. Generally, a lowerresistance provided by the control circuit 700 effectively results in ahigher trigger voltage than for a higher control circuit resistance. Asa result, the resistance between the p-well and the reference voltagecan be modulated to adjust the snapback performance characteristics ofthe protection circuit.

For example, in embodiments of the invention utilizing the protectioncircuit 500 (FIG. 5) and the control circuit 700, the CNTRL signal canbe used to in effect modulate trigger and hold conditions for theprotection circuit 500. Operation of the protection circuit and thecontrol circuit 700 will be made with reference to FIG. 3.

Under a first condition with CNTRL=V1 (e.g., V1<Vt of the transistor720) the transistor 720 behaves as an open circuit, thereby adding theRcontrol resistance to the total resistance between the p-well andreference voltage of resistance 710. As a result, the p-well currentrequired to forward bias the base-to-emitter of the NPN-BJT 420 isdecreased, thereby effectively decreasing the trigger voltage of theprotection circuit 500. In some embodiments, the resistance 710 is arelatively high resistance, for example, 50-100 kohms.

In contrast, under a second condition with CNTRL=V2 (e.g., V2>Vt), theresistance of the control circuit 700 will be less than Rcontrol of theresistance 710. With the lower resistance, the trigger voltage for theprotection circuit 500 effectively increases. In a condition where theCNTRL signal is high enough to cause the transistor 720 to have a lowresistance, for example, around 100 ohms, which results in essentiallyelectrically shorting the p-well to ground, the protection circuit 500will exhibit performance characteristics similar to having anun-modulated shunt circuit 510.

FIG. 8 illustrates a protection circuit 800 according to an embodimentof the invention. The protection circuit 800 includes similar componentsas the protection circuit 500 previously described with reference toFIG. 5. The same reference numbers used for the embodiment of FIG. 5 areused in FIG. 8 where applicable. The protection circuit 800 furtherincludes a diode 820 coupled between the cathode node 30 and the pad 20in contrast to the protection circuit 500. The cathode node 30 isillustrated in FIG. 8 as being coupled to a reference voltage, forexample, ground. The diode 820 may be a diode configured to provide ESDprotection for ESD events that cause current to conduct from the cathodenode 30 to the pad 20. As a result, the protection circuit 800 providescurrent conduction during over-voltage conditions from the pad 20 to thecathode node 30 (i.e., current I1) as well as from the cathode node 30to the pad 20 (i.e., current I2). With reference to the cross-sectionalview of FIG. 6, the diode 820 may be formed in n-well 608 by furtherforming a p-region in the n-well 608 to provide the anode of diode 820.The p-region may be coupled to the cathode node 30.

FIG. 9 illustrates a protection circuit 900 according to an embodimentof the invention. The protection circuit 900 includes a first protectioncircuit 910 coupled to a pad 20 and a cathode node 30, shown in FIG. 9as being coupled to a reference voltage node (e.g., ground). Theprotection circuit 900 further includes a second protection circuit 920coupled to the pad 20 and a power supply node 40 which may be coupled toa power supply, for example, Vcc. Generally, the voltage coupled to thepower supply node 40 is less than the voltage provided to the pad 20. Aprotected circuit 10 is coupled to the pad 20 and the cathode node 30.The first protection circuit 910 is configured to conduct current fromthe pad 20 to the cathode as well as conduct current from cathode node30 to the pad 20. The first protection circuit 910 may be implementedusing the protection circuit 800 of FIG. 8, which includes a diodeconfigured to provide ESD protection for ESD events that cause currentto conduct from the cathode node 30 to the pad 20. The second protectioncircuit 920 is configured to conduct current from the pad 20 to thepower supply node 40. The second protection circuit 920 may beimplemented using the protection circuit 500 of FIG. 5. The protectioncircuit 900 may be used to provide ESD protection for both power andground sides.

FIG. 10 illustrates a portion of a memory 1000 according to anembodiment of the present invention. The memory 1000 includes an array1002 of memory cells, which may be, for example, DRAM memory cells, SRAMmemory cells, flash memory cells, or some other types of memory cells.The memory 1000 includes a command decoder 1006 that receives memorycommands through a command bus 1008 and generates corresponding controlsignals within the memory 1000 to carry out various memory operations.Row and column address signals are applied to the memory 1000 through anaddress bus 1020 and provided to an address latch 1010. The addresslatch then outputs a separate column address and a separate row address.

The row and column addresses are provided by the address latch 1010 to arow address decoder 1022 and a column address decoder 1028,respectively. The column address decoder 1028 selects bit linesextending through the array 1002 corresponding to respective columnaddresses. The row address decoder 1022 is connected to word line driver1024 that activates respective rows of memory cells in the array 1002corresponding to received row addresses. The selected data line (e.g., abit line or bit lines) corresponding to a received column address arecoupled to a read/write circuitry 1030 to provide read data to a dataoutput buffer 1034 via an input-output data bus 1040. The output buffer1034 may include output driver circuits (not shown) coupled to pad 1020to be provided a relatively high voltage when providing output data.Write data are applied to the memory array 1002 through a data inputbuffer 1044 and the memory array read/write circuitry 1030. The commanddecoder 1006 responds to memory commands applied to the command bus 1008to perform various operations on the memory array 1002. In particular,the command decoder 1006 is used to generate internal control signals toread data from and write data to the memory array 1002.

Over-voltage/over-current protection circuit 1050 according to anembodiment of the present invention is coupled to the pad 1020. Theprotection circuit 1050 protects circuits of the output buffer 1034 thatare coupled to the pad 1020 (e.g., driver circuits) in the event arelatively high-voltage/high-current signal is applied to the pad 1020.Additionally, as previously discussed, the protection circuit 1050allows for modulating the trigger conditions and the hold conditions forthe protection circuit. In some embodiments, the protection circuit maybe used in power-up sequences for the memory 1000, as previouslydiscussed. That is, while no power is applied to the memory 1000, thetrigger conditions for the protection circuit 1050 is relatively low. Incontrast, after power has been applied to the memory 1000, the triggerconditions for the protection circuit 1050 is modulated to a highertrigger condition relative to when no power is applied.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. An apparatus, comprising: a trigger circuitcoupled to a first node and to a protected circuit, the trigger circuitfurther configured to provide a leakage current responsive to a voltageprovided to the first node exceeding a trigger condition, wherein thetrigger circuit includes a plurality of series coupled transistorscoupled between the first node and a second node and a transistor of theplurality of series coupled transistors is formed in a semiconductordoped well region in which at least a portion of the shunt circuit isformed; and a shunt circuit configured to conduct current from the firstnode to the second node responsive to the provided leakage current. 2.The apparatus of claim 1 wherein the shunt circuit comprises a circuithaving a snapback electrical characteristic.
 3. The apparatus of claim 1wherein the shunt circuit comprises a silicon controlled rectifier. 4.The apparatus of claim 1 wherein the trigger circuit comprises a circuitthat is matched to the protected circuit.
 5. The apparatus of claim 1wherein the trigger condition comprises a voltage being applied to thefirst node exceeding a trigger voltage of the trigger circuit.
 6. Theapparatus of claim 1, further comprising a memory array.
 7. Anapparatus, comprising: a protected circuit coupled to a first node and asecond node; and a protection circuit coupled to the protected circuitand configured to conduct current from the first node to the second nodeto protect the protected circuit from an over-limit electricalcondition, the protection circuit having a trigger circuit coupled tothe first node and configured to trigger a shunt circuit to conductcurrent from the first node to the second node responsive to a voltageprovided to the first node having a voltage exceeding a trigger voltage,the trigger circuit matching the protected circuit.
 8. The apparatus ofclaim 7 wherein the protection circuit includes a control circuitcoupled to the shunt circuit, the control circuit configured to adjustthe trigger voltage.
 9. The apparatus of claim 8 wherein the controlcircuit is configured to change the triggering of the shunt circuitrelative to the voltage provided to the pad node responsive to a controlsignal.
 10. The apparatus of claim 8 wherein the control circuitcomprises: an adjustable resistance coupled to a semiconductor dopedregion in which at least a portion of the shunt circuit is formed. 11.The apparatus of claim 10 wherein the adjustable resistance comprises: aresistance; and a switch coupled in parallel with the resistance, theswitch configured to be opened and closed responsive to a controlsignal.
 12. The apparatus of claim 8 wherein the control circuit sets afirst trigger voltage for the trigger circuit during power-up and sets asecond trigger voltage lower than the first trigger voltage followingpower-up.
 13. The apparatus of claim 7 wherein the protection circuitfurther comprises a diode coupled to the first node and the second node,the diode configured to provide a current path from the second node tothe first node.
 14. The apparatus of claim 7 wherein the protectioncircuit is a first protection circuit, and the apparatus furthercomprises a second protection circuit coupled to the first node and athird node and including a base modulated silicon controlled rectifier,the second protection circuit configured to conduct current between thefirst node and the third node responsive to a voltage provided to atleast one of the first node and the third node exceeding a triggervoltage of the second protection circuit.
 15. The method of claim 7wherein the shunt circuit comprises a thyristor.
 16. An apparatus,comprising: a trigger circuit coupled to a protected circuit and to afirst node and to a second node, the trigger circuit configured toprovide a leakage current responsive to a voltage provided to the firstnode exceeding a trigger circuit trigger voltage; a diode having acathode coupled to the first node; and a base modulated siliconcontrolled rectifier coupled to receive the leakage current and coupledto the first node and to the second node, the base modulated siliconcontrolled rectifier further configured to conduct current from thefirst node to the second node responsive to the leakage current, atleast a portion of the trigger circuit and at least a portion of thebase modulated silicon controlled rectifier formed in a commonsemiconductor doped region.
 17. The apparatus of claim 16 wherein thebase modulated silicon controlled rectifier comprises: a PNP bipolarjunction transistor having a emitter coupled to a base; and an NPNbipolar junction transistor coupled in series with the PNP bipolarjunction transistor.
 18. The apparatus of claim 17 wherein the base anda collector of the PNP bipolar junction transistor are substantially thesame as a collector and base of the NPN bipolar junction transistor. 19.The apparatus of 17 wherein the transistor of the trigger circuit sharesa semiconductor doped region with an emitter of the NPN bipolar junctiontransistor.
 20. The apparatus of claim 16 wherein the base modulatedsilicon controlled rectifier includes a portion formed in asemiconductor doped well region in which at least a portion of thetrigger circuit is also formed.
 21. A method, comprising: generating aleakage current responsive to a voltage being provided to a first nodethat exceeds a trigger voltage, wherein the leakage current is providedby a transistor having a gate coupled to a gate of a transistor of aprotected circuit; and triggering a circuit to conduct current from thefirst node to a second node responsive to the leakage current byincreasing a voltage of a semiconductor doped well using at least aportion of the leakage current.
 22. The method of claim 21 whereintriggering the circuit to conduct current comprises forwarding biasing abase of a silicon controlled rectifier.
 23. The method of claim 21,further comprising setting the trigger voltage to a first triggervoltage during power-up and setting the trigger voltage to a secondtrigger voltage during normal operations, the second trigger voltagehigher than the first trigger voltage.
 24. A method, comprising: settinga trigger voltage to a first trigger voltage during power-up and settingthe trigger voltage to a second trigger voltage during normaloperations, the second trigger voltage higher than the first triggervoltage; generating a leakage current responsive to a voltage beingprovided to a first node that exceeds the trigger voltage, wherein theleakage current is provided by a transistor having a gate coupled to agate of a transistor of a protected circuit; and triggering a circuit toconduct current from the first node to a second node responsive to theleakage current.